Fuel burner safety control circuit

ABSTRACT

A fuel burner safety control circuit for burner safety shutoff valves functioning in different operational modes depending upon the occurrence of an interruption in power to the circuit or an unsafe operating condition in the overall system so as to be capable of distinguishing between a momentary power interruption where the burner should not be shut down and an emergency condition where the burner should be shut down substantially immediately. The safety control circuit has a deenergize-to-trip burner safety shutoff valve with an integral time delay to maintain burner safety shutoff valve energization during momentary power interruption, and a bypass arrangement permitting the time delay to be bypassed during emergency trip conditions to provide substantially immediate burner safety shutoff valve deenergization for closing down the burner system. This capability of distinguishing between momentary power interruptions and emergency trip conditions is accomplished by using a circuit having parallel time delay relays to activate switches set in series in a time delay circuit. The time delay relays function in opposite energization senses (i.e., energize to trip and deenergize to trip) in activating the switch with which it is associated, one switch being normally closed and the other closed on application of power to the safety control circuit. On a power interruption, both relays would assume a deenergized state maintaining the switches closed for a set period of time to continue the integrity of the time delay circuit to prevent trip of the deenergize-to-trip burner safety shutoff valve. This is accomplished by the deenergize-to-trip relay holding in the powered closed switch for a discrete period of time and the energize-to-trip relay (being deenergized) having no effect on the normally closed switch. In an emergency unsafe operating condition the deenergize-to-trip relay would again be deenergized to maintain the powered closed switch closed, but the energize-to-trip relay would assume an energized state substantially immediately opening the normally closed switch to trip the deenergize-to-trip burner safety shut off valve for closing down the burner system.

United States Patent [72] Inventors Thomas W. Brown Vernon; Michael A.Delin, West Simsbury; Robert D. Scoville, Torrington, all of Conn. [21]Appl. No. 45,369 [22] Filed June 11, 1970 [45] Patented Nov. 9, 1971[73] Assignee Combustion Engineering, Inc.

Windsor, Conn.

[54] FUEL BURNER SAFETY CONTROL CIRCUIT 7 Claims, 4 Drawing Figs.

[52] U.S.Cl 317/36 TI), 251/129, 307/116,307/117 [51] Int. Cl. 1102b3/24 [50] Field of Search 317/36,40; 307/1 16, 117, 118,236/1 A, I H;431/66, 69, 18; 137/94; 251/129 [56] References Cited UNITED STATESPATENTS 3,254,871 6/1966 Limon 431/18 X 3,275,058 9/1966 Gage 431/69 XPrimary ExaminerJ. D. Miller Assistant ExaminerHarvey FendelmanAttorneys-Carlton F. Bryant, Eldon H. Luther, Robert L. Olson, John F.Carney, Richard H. Bemeike, Edward L. Kochey, Jr. and Lawrence P.Kessler ABSTRACT: A fuel burner safety control circuit for burner safetyshutoff valves functioning in different operational modes depending uponthe occurrence of an interruption in power to the circuit or an unsafeoperating condition in the overall system so as to be capable ofdistinguishing between a momentary power interruption where the burnershould not be shut down and an emergency condition where the burnershould be shut down substantially immediately. The safety controlcircuit has a deenergize-to-trip burner safety shutoff valve with anintegral time delay to maintain burner safety shutoff valve energizationduring momentary power interruption, and a bypaw arrangement permittingthe time delay to be bypassed during emergency trip conditions toprovide substantially immediate burner safety shutoff valvedeenergization for closing down the burner system. This capability ofdistinguishing between momentary power interruptions and emergency tripconditions is accomplished by using a circuit having parallel time delayrelays to activate switches set in series in a time delay circuit. Thetime delay relays function in opposite energization senses (i.e.,energize to trip and deener' gize to trip) in activating the switch withwhich it is associated, one switch being normally closed and the otherclosed on application of power to the safety control circuit. On a powerinterruption, both relays would assume a deenergized state maintainingthe switches closed for a set period of time to continue the integrityof the time delay circuit to prevent trip of the deenergize-to-tripburner safety shutoff valve. This is accomplished by thedeenergize-to-trip relay holding in the powered closed switch for adiscrete period of time and the energize-to-trip relay (beingdeenergized) having no efiect on the normally closed switch. In anemergency unsafe operating condition the deenergize-to-trip relay wouldagain be deenergized to maintain the powered closed switch closed, butthe energize-to-trip relay would assume an energized state substantiallyimmediately opening the normally closed switch to trip thedeenergize-to-trip burner safety shut ofi valve for closing down theburner system.

FUEL BURNER SAFETY CONTROL CIRCUIT BACKGROUND OF THE INVENTION Thecomplexity of the modern power generating facility requires thatautomatic systems be employed for control and safety purposes. Suchautomatic systems are necessary to assure that combustion rate isaccurately matched to power demand for maximum plant efiiciency and thatcertain operating conditions are not exceeded for overall systemprotection. Automatic limit controls for safety purposes can, in and ofthemselves, however, create safety problems.

This is particularly true with regard to burner arrangements whereinshutdown and reignition presents a potentially explosive condition dueto unburned fuel accumulation in the fur nace. It is, therefore,necessary to provide an automatic burner control arrangement which iscapable of distinguishing between an emergency condition whereinsubstantially immediate system shutdown is necessary and a transientcondition which appears to exceed safe operating limits but whereindelayed action for a specific period may be tolerated to permit normalconditions to be reestablished. One such transient condition which givesa false signal of shutdown necessity may occur when electrical powerinput to the burner control system is switched from one power bus toanother. Presently used methods to prevent shutdown of burner controlsystems due to transients include using continuous DC, using an inverterwith zero transfer time, using a capacitor bank, or employingenergize-to-trip systems (no trip on power loss possi ble).

SUMMARY OF THE INVENTION It is the purpose of this invention toaccomplish control of a fuel burner system which is capable ofdistinguishing between certain transient conditions, such as theabove-noted momentary power interruptions, which give a false indicationof the necessity for system shutdown and true emergency conditions, andoperate in a more efficient and economical manner than that of theabove-noted methods. There is herein provided a safety control circuitwhich relies upon a deenergize-to-trip burner safety shut off valve withan integral time delay to maintain valve energization during a momentarypower interruption to prevent burner shutdown. The safety controlcircuit has a bypass feature which permits the time delay to be bypassedduring emergency trip conditions to provide substantially immediateburner safety shut off valve deenergization and resulting burner systemshutdown. The capability of distinguishing between momentary powerinterruptions and emergency trip conditions is accomplished by using acircuit having parallel time delay relays to activate switches set inseries in a time delay circuit. The time delay relays function inopposite energiration senses (i.e., energize to trip and deenergize totrip) in activating the switch with which it is associated, one switchbeing normally closed and the other closed on application of power tothe safety control circuit. On a power interruption both relays wouldassume a deenergized state maintaining the switches closed for a setperiod of time to continue the integrity of the time delay circuit toprevent trip of the deenergize-to-trip burner safety shut off valve.This is accomplished by the deenergize-to-trip relay holding in thepowered closed switch for a discrete period of time and theenergize-totrip relay (being deenergized) having no effect on thenormally closed switch. Thus, if power loss is momentary, circuitintegrity is maintained and burner shutdown is avoided; however, ifpower loss exceeds the set period of time, the powered closed switchwill open deenergizing the deenergize-to-trip burner safety shut offvalve closing down the burner system. In an emergency unsafe operatingcondition, the deenergize-totrip relay would again be deenergized tomaintain the powered closed switch closed, but the energize-to-triprelay would assume an energized state substantially immediately openingthe normally closed switch to trip'the deenergize-to-trip burner safetyshut off valve for closing down the burner system. The energize-to-triprelay does have a slight time delay action in order to preventaccidental tripping of the normally closed valve on resumption of powerafter a momentary power interruption.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof the fuel burner safety control circuit according to this invention.

FIG. 2 is a schematic representation of the novel safety control circuitof FIG. 1 showing the circuit in its armed, poweron mode.

FIG. 3 is a schematic representation of the novel safety control circuitof FIG. 1 in its power-interruption mode.

FIG. 4 is a schematic representation of the novel safety control circuitof FIG. 1 in its emergency trip mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,FIG. 1 is a schematic representation of the fuel burner safety controlcircuit 10 according to this invention. The circuit 10 comprises leadlines 12, 12' across which an AC power supply (not shown) is located.Between the lead lines 12, 12' there is a number of parallel paths forthe circuit elements. It is noted that the legend identifies thesymbolsused throughout the drawings representing various elements withinthe circuit 10.

The first parallel path comprises a relay actuated contact switch 14which is a control switch dependent upon satisfactory accomplishment ofnecessary boiler operating conditions, such as feedwater flow and properfan operation, for actuation thereof. In series therewith is a boilerpurge control relay actuated contact switch 16 and a master fuel triprelay l8. Across the switch 16, so as to be in parallel relationshipwith it, is a master fuel trip locking switch 20. A first time delayrelay 22 is located in parallel with the master fuel trip relay l8 andin series with the contact switches 16 and 20. Additionally, a timedelay switch 24, in series with a proof of trip safety shut off valveopen contact switch 25, is in parallel with the master fuel trip lockingswitch 20 for the purposes to be explained hereinbelow.

The second parallel path includes a first master fuel trip relayactuated contact switch 26 which is maintained in a normally closedposition. A second time delay relay 28 is in series with switch 26,which, of course, controls the energization of the relay 28.

The third parallel path of the circuit 10 includes a relay ac tuatedcontact switch 30 which is a control switch dependent upon burneroperating conditions, such as fuel availability and power for ignition,for the actuation thereof. In series therewith is a second master fueltrip relay actuated contact switch 32 and a deenergize-to-trip burnersafety shut off valve 34 for shutting off fuel to the burner system. Aburner safety shut off valve time delay loop 36 is in parallel with theburner safety shut off valve 34. The loop 36 consists of a first timedelay switch 40 actuated by relay 22 (upon deenergization thereof) and asecond time delay switch 38 actuated by relay 28 (upon energizationthereof). In series with the two time delay switches 38 and 40 is a tripdelay unit 42, for the purpose of maintaining power supply to the burnersafety shut off valve 34 for a discrete period of time, the unit 42being of any well-known type such as the Maxon "I'D trip delay unit madeby the Mason Premix Burner Company Inc.

The above-described elements of the safety control circuit 10 areinterrelated to operate in the following manner. An AC power source isconnected across the leads l2, l2 and when energized the circuit isarmed leading to the mode as represented in FIG. 2 in the followingsequence. Upon satisfactory accomplishment of all necessary boileroperating conditions, the switch 14 will be actuated to its closedposition. With all boiler operating conditions satisfied, the boiler ispurged of latent gases by forcing of air therethrough for a period oftime. The purge control relay actuated switch 16 is closed at completionof satisfactory boiler purge (switch 16 remains closed only for a shortperiod of time) to energize the master fuel trip relay l8 and the firsttime delay relay 22. The master fuel trip relay 18 in turn actuates themaster fuel trip locking switch 20 to a closed position and at the sametime serves to open the first master fuel trip relay actuated switch 26while closing the second master fuel trip relay activated switch 32.With the switch 26 in its open position, power is prevented from flowingthrough the second parallel path. The second time delay relay 28 is thusmaintained in a deenergized state which in turn maintains the switch 38which it controls in its normally closed position.

At the same instant at which power is supplied to the first time delayrelay 22, the relay 22 actuates the first time delay relay lockingswitch 24 and first time delay switch 40 to closed positions. The firsttime delay relay switch 24, in series with the proof of trip burnersafety shut off valve open contact switch 25, serves to maintain circuitintegrity and a power path for relays l8 and 22 upon momentary powerinterruptions. The switch 25 permits this circuit integrity to existonly when the burner safety shut off valve is open (and it iscorrespondingly closed).

After the purge cycle is completed and when all necessary burneroperating conditions have been met, the relay actuated contact switch 30will be closed to power the deenergize-totrip burner safety shut offvalve 34 to its active (open) position, the proof of trip safety shutoff valve open contact switch 25 simultaneously being closed. At thesame time, the safety shut off valve time delay loop 36 will be broughtto an energized state due to the fact that the first and second timedelay switches (40, 38) are in their closed positions.

Upon an electrical power interruption to the burner control system, thenovel safety control circuit is activated to the mode shown in FIG. 3.Power across leads 12, 12' ceases caus ing the boiler operatingcondition switch 14 and the burner operating condition switch 30 toreturn to their open positions which would ordinarily deenergize themaster fuel trip relay 18, first time delay relay 22 and thedeenergize-to-trip burner safety shut off valve 34. However, thedeenergize-to-trip burner safety shut off valve is maintained energizedby the trip delay unit 42 in the time delay loop 36 for a period of timein the following manner. The first time delay relay 22, upondeenergization thereof, will maintain the time delay switch 24 and firsttime delay switch 40 of the time delay loop 36 closed for a discreteperiod of time (e.g., one second). This will retain the integrity of theelectrical path of the burner safety shut off valve time delay loop 36so that energy stored in the time delay unit 42 may maintain the burnersafety shut off valve 34 in an energized state so as to prevent thetripping thereof and resultant burner shutdown.

The purpose of maintaining switch 24 in its closed position for thediscrete period of time is to provide a path for power through themaster fuel trip relay l8 and time delay relay 22 upon the resumptionthereof. The switches 24 and 40 as noted remain closed only for a periodof time so as to compensate for transient power interruptions. That isto say, if the power interruption is only momentary, thedeenergize-to-trip burner safety shut off valve 34 will be maintained inits energized state (by trip delay unit 42) and the circuit will remainarmed so that upon the return of power, the system will return to itsmode shown in FIG. 2. if, however, the power interruption is nottransient but is of a significant time duration, the switches 24 and 40will open thus deactivating the deenergize-to-trip burner safety shutoff valve 34 closing down the burner operations and dearrning the safetycontrol circuit 10.

Under certain circumstances, an emergency condition may arise at whichtime the deenergize-to-trip burner safety shut off valve 34 must besubstantially immediately deenergized to shut down burner operationrather than permitting an extended delay as above. The emergencycondition may either occur in the burner operating conditions such asflame failure or the boiler operating conditions such as feedwaterinterruption. The burner shutdown is accomplished by activating circuit10 to the mode represented in FIG. 4 in the following manner. Upon theoccurrence of an emergency condition, the

switches 14 (or switch 14 in cooperation with switch 30 if the emergencyis in the burner system) will be actuated to its open position, thuscutting off power to the master fuel trip relay l8 and the first timedelay relay coil 22. As a result thereof, second master fuel trip relayactuated switch 32 will open interrupting power to thedeenergize-to-trip burner safety shut off valve 34. As in the powerinterruption mode represented in FIG. 3, the deenergization of the firsttime delay relay 22 will cause the switches 24 and 40 to remain in theirclosed positions to preserve circuit integrity. This, of course, willmaintain the energization of the burner safety shut off valve time delayloop 36 by the trip delay unit 42.

in order to promptly interrupt this energization and cause the valve 34to trip to shut down the burner, the switch 38 is substantiallyimmediately opened in the following manner. Upon the opening of switch14 and the deenergization of master fuel trip relay 18, the first masterfuel trip relay actuated switch 26 is returned to its normally closedposition. Since the power supply between the leads l2 and 12' is stillbeing maintained, the path through the first master fuel trip relayactuated switch 26 will be powered to energize the second time delayrelay 28. Upon energization thereof, the second time delay switch 38 inthe time delay loop 36 will be maintained closed for a discrete periodof time (e.g., onetenth of a second) and then will open to deenergizethe deenergize-to-trip burner safety shut off valve 34 so as tosubstantially immediately shut down burner operation. The small timedelay in the opening of switch 38 by the time delay relay 28 (as opposedto an instantaneous response) is necessary for proper rearming of theburner safety control circuit after a momentary power interruption. Thetime delay permits the circuit to return to the mode represented in FIG.2 after the momentary power interruption by keeping switch 38 closeduntil the second time delay relay 28 is deenergized. if the time delaywere not present, upon resumption of power, the second time delay relay28 would be momentarily energized (thus opening switch 38 disrupting theintegrity of time delay loop 36) before it could be deenergized byopening of switch 26 by master fuel trip relay 18.

From the foregoing it is apparent that there is herein described a novelfuel burner safety control circuit operating so as to be capable ofdiscriminating between transient power interruptions where burnershutdown is not desirable and actual emergency conditions wheresubstantially immediate burner shutdown is necessary. This isaccomplished by constructing a control circuit with parallel time delayrelays functioning in opposite energization senses to activate seriesswitches in a time delay loop in parallel with a deenergize-to tripburner safety shut off valve. Upon a power interruption, both relays aredeenergized maintaining the switches closed for a set period of timepermitting a time delay device in the loop to maintain the safety shutoff valve energized; under emergency conditions, one relay will bedeenergized while the other will be energized opening one of theswitches in the time delay loop, thus deenergizing the loop in order toprovide substantially immediate tripping of the burner safety shut offvalve and corresponding burner shutdown.

While this preferred embodiment of the invention has been shown anddescribed, it will be understood that it is merely illustrative and thatchanges may be made without departing from the scope of the invention asclaimed.

We claim:

1. A fuel burner safety control circuit operable so as to be capable ofdistinguishing between power interruptions and emergency operationconditions necessitating closing a burner safety shut off valve, saidsafety control circuit including: a deenergize-to-trip burner safetyshut off valve; a power source across said burner safety shut off valveto energize said valve; a time delay loop in parallel relationship tosaid burner safety shut off valve, said time delay loop having a timedelay unit and a first and second switch in series relationship with oneanother; a first time delay relay operatively associated with said firstswitch to maintain said switch closed for a discrete period of time upondeenergization of said relay; a second time delay relay operativelyassociated with said second switch to maintain said switch closed for adiscrete period of time upon energization of said relay, said discreteperiod of said second switch being substantially less than said discreteperiod of said first switch; and means connected with said power sourcefor controlling the energization state of said first and second timedelay relays so that both time delay relays are deenergized on powerinterruption and during an emergency condition said first time delayrelay is deenergized and said second time delay relay is energized.

2. The safety control circuit of claim 1 wherein said means forcontrolling the energization state of said first and second time delayrelays includes a first control switch in series relationship with saidfirst time delay relay, a master fuel trip relay in series relationshipwith said first control switch and in parallel relationship with saidfirst time delay relay, and a second control switch in seriesrelationship with said second time delay relay, said second controlswitch operatively associated with said master fuel trip relay so as tobe activated to an open position from a normally closed position uponactivation of said master fuel trip relay through said first controlswitch.

3. The safety control circuit of claim 2 wherein said first controlswitch is dependent upon satisfaction of all necessary boiler operatingconditions before actuation thereof to a closed position is initiated.

4. The safety control circuit of claim 2 wherein there is furtherincluded a third control switch to selectively permit initialenergization of said deenergize-to-trip burner safety shut off valve,activation of said third control switch to its closed position beingdependent upon satisfaction of all necessary burner operatingconditions.

5. The safety control circuit 'of claim 2 wherein there is provided alocking switch in series between said first control switch and saidmaster fuel trip relay, said locking switch operatively associated withsaid first time delay relay to remain closed for a discrete period oftime to maintain integrity of the power path to said master fuel triprelay.

6. The safety control circuit of claim 1 wherein said discrete period oftime that said first switch remains closed is on the order of 10 timesthe discrete period of time that said second switch remains closed.

7. The safety control circuit of claim 5 wherein said discrete period oftime that said first switch remains closed is one second and saiddiscrete period of time that said second switch remains closed is 0.10second.

1. A fuel burner safety control circuit operable so as to be capable ofdistinguishing between power interruptions and emergency operationconditions necessitating closing a burner safety shut off valve, saidsafety control circuit including: a deenergize-to-trip burner safetyshut off valve; a power source across said burner safety shut off valveto energize said valve; a time delay loop in parallel relationship tosaid burner safety shut off valve, said time delay loop having a timedelay unit and a first and second switch in series relationship with oneanother; a first time delay relay operatively associated with said firstswitch to maintain said switch closed for a discrete period of time upondeenergization of said relay; a second time delay relay operativelyassociated with said second switch to maintain said switch closed for adiscrete period of time upon eNergization of said relay, said discreteperiod of said second switch being substantially less than said discreteperiod of said first switch; and means connected with said power sourcefor controlling the energization state of said first and second timedelay relays so that both time delay relays are deenergized on powerinterruption and during an emergency condition said first time delayrelay is deenergized and said second time delay relay is energized. 2.The safety control circuit of claim 1 wherein said means for controllingthe energization state of said first and second time delay relaysincludes a first control switch in series relationship with said firsttime delay relay, a master fuel trip relay in series relationship withsaid first control switch and in parallel relationship with said firsttime delay relay, and a second control switch in series relationshipwith said second time delay relay, said second control switchoperatively associated with said master fuel trip relay so as to beactivated to an open position from a normally closed position uponactivation of said master fuel trip relay through said first controlswitch.
 3. The safety control circuit of claim 2 wherein said firstcontrol switch is dependent upon satisfaction of all necessary boileroperating conditions before actuation thereof to a closed position isinitiated.
 4. The safety control circuit of claim 2 wherein there isfurther included a third control switch to selectively permit initialenergization of said deenergize-to-trip burner safety shut off valve,activation of said third control switch to its closed position beingdependent upon satisfaction of all necessary burner operatingconditions.
 5. The safety control circuit of claim 2 wherein there isprovided a locking switch in series between said first control switchand said master fuel trip relay, said locking switch operativelyassociated with said first time delay relay to remain closed for adiscrete period of time to maintain integrity of the power path to saidmaster fuel trip relay.
 6. The safety control circuit of claim 1 whereinsaid discrete period of time that said first switch remains closed is onthe order of 10 times the discrete period of time that said secondswitch remains closed.
 7. The safety control circuit of claim 5 whereinsaid discrete period of time that said first switch remains closed isone second and said discrete period of time that said second switchremains closed is 0.10 second.